1. Field of the Invention
The invention disclosed and claimed herein is in the field of semiconductor devices and processes for the manufacture thereof. Specifically, the invention is directed toward the preparation of light emitting diodes (LED's) by the epitaxial growth of gallium phosphide (GaP) on silicon (Si) substrates.
2. Prior Art
The literature is replete with reports covering heteroepitaxial deposition studies encompassing various types of film-substrate systems and the structures produced have been generally intended for use in further examination of heterojunction devices such as heterodiodes, or for the fabrication of devices onto the epitaxial film. In the latter case, the substrate is normally chosen on a basis of economics or because of certain properties and tends to act merely as a support for the film. One potentially fruitful area of heteroepitaxial study is that of epitaxial LED material deposited on silicon substrates, primarily because of the wide availability of large area, high quality and relatively low cost silicon substrates. A series of different deposition processes have been attempted to achieve monolithic integration of GaP LED's on silicon substrates.
The halide transport process [Ga-PCl.sub.3 -(88% H.sub.2, 12 % H.sub.2)] was used by Kesperis et al and reported June, 1964 [Technical Report ECOM-2471, "Research on Heterojunctions", U.S. Army Electronics Command]. Difficulties, mainly due to the formation of SiO.sub.2 on the silicon substrate, were experienced. The typical growth temperature was 700.degree.-750.degree.C and the utilization of slow growth rates, on the order of 0.7 .mu./min. were found to retard film cracking in layers up to 22 .mu. thick. A slightly different halide transport system, employing GaP and PCl.sub.3 as the source materials, has been recently reported by Huber and Winstel ["Growth of Heteroepitaxial GaP on Si Substrates by a Chloride Transport Process", Siemens Forsch-n. Entwickl - Ber. Bd 2(1973), No. 3]. This research also experienced deposition difficulties due to the presence of oxide on the silicon substrates and successful depositions, at about 800.degree.-850.degree.C, were achieved on &lt;110&gt; substrates but not on &lt;100&gt;substrates provided the silicon substates underwent a specific high temperature H.sub.2 bake cycle.
The use of a close spaced evaporation technique has been disclosed by Igarashi ["Heteroepitaxial Growth of GaP and Si Substrates by Evaporation Method", J. Applied Physics 41, 3190 (1970); "Selective Growth of Heteroepitaxial GaP on Si Substrates", J. Electrochem Soc., Vol. 119, p. 1430, (1972)]. Depositions into SiO.sub.2 holes (about 70 .times. 200 .mu.) were crack free, so long as the depositions were less than 5 .mu. thickness. Also, Thomas ]"Growth of Single Crystal GaP from Organometallic Sources", J. Electrochem Soc., Vol. 116, No. 10, p. 1449 (1969)] has reported on the pyrolysis technique at the lowest reported growth temperature (485.degree.C). However, the growth rate (0.0025 .mu./min.) and film thickness (0.5 .mu.) were of little practical use for GaP diode applications.
Other process methods such as closed tube transport [Hoack and Mohling, "Epitaxial Layers of Gallium Phosphide on Silicon", Phys. Stat. Sol. 3A K229 (1970)], eutectic growth [Rosztoczy and Stein, "The Growth of Ge-GaAs and GaP-Si Heterojunctions by Liquid Phase epitaxy", J. Electrochem Soc., Vol. 119, No. 8, p. 1119 (1972)] and electrolytic depositions [Cuomo and Gambins, "The Synthesis and Epitaxial Growth of GaP by Fused Salt Electrolysis", J. Electrochem Soc., Vol. 115, No. 7 (1967)] have been used to study the GaP on Si heterosystem, but these tend not to lend themselves very readily to any large scale, practical production methods.
Similarly, prior art patents such as Ruerwein, U.S. Pat. No. 3,312,570; LaChapelle, U.S. Pat. No. 3,582,410; and Yu, U.S. Pat. No. 3,366,517 relate to studies for establishing appropriate conditions for achieving heteroepitaxial growths. Very few material characteristics have been reported in the prior art because most material was not suitable for such measurement and analysis. The available data is restricted to commonly observed film cracking along cleavage planes, Hall measurements and cursory structural and optical data. The patent to Mason, U.S. Pat. No. 3,766,447 attempts to overcome the problem of the mismatch between Si and GaP by the use between the two of a graded alloy layer consisting of silicon with a germanium (Ge) concentration ranging from zero at the junction with the Si substrate to about 8% at the junction with the GaP layer. The prior art as represented by Mason moves away from the desired goal of direct epitaxial deposition of an electroluminescent semiconductor on silicon in attempts to achieve a commerically viable product by compromising, in the process, characteristics of the materials to reach a satisfactory result.